Replicas have physical clocks, which are loosely synchronized by a
protocol such as NTP. The correctness of UniStore does not depend on the
precision of clock synchronization, but large drifts may negatively impact its performance.
Any two replicas are connected by a reliable FIFO channel, so that
messages between correct data centers are guaranteed to be delivered.
As is standard, to implement strong consistency we require the network to be
eventually synchronous, so that message delays between sibling replicas in correct
data centers are eventually bounded by some constant [1]}.
